A conventional quadrature modulation circuit includes a divider for dividing a local signal into two local signals, a phase shifter for shifting a phase of one of the divided local signals whereby two orthogonal local signals having a relative phase difference of 90 degrees are obtained, two mixers for combining two baseband signals with the orthogonal local signals, respectively, and an adder for adding output signals of the mixers. The output signal of the quadrature modulation circuit is an RF (radio frequency) signal thus modulated to be transmitted to another station.
According to the conventional quadrature modulation circuit, however, there is a disadvantage/in that the shifting amount of the phase shifter is difficult to be maintained at 90 degrees precisely, because the shifting amount is affected by a printed pattern connected to the phase shifter and the like. As a result, the characteristics of a quadrature modulation will be deteriorated.